Valve apparatus and hydraulic drive system

ABSTRACT

A valve apparatus has a flow control valve having a supply passage communicating with a hydraulic fluid supply source, a load passage communicating with an actuator, and a first meter-in variable restrictor disposed between the supply passage and the load passage and opened dependent on an operation amount there; a first signal passage located downstream of the first variable restrictor and having a passage section for detecting load pressure of the actuator; a tank passage communicating with a reservoir tank; a discharge passage for communicating the first signal passage with the tank passage; and a second variable restrictor provided in the discharge passage and having its opening variable dependent on the operation amount of the flow control valve to produce in the first signal passage a control pressure different from the load pressure, the control pressure in the first signal passage being led to the hydraulic fluid supply source though a second signal passage. An auxiliary restrictor is disposed in the first signal passage to reduce the load pressure detected in the passage section of the first signal passage so that a pressure lower than the detected load pressure is produced in the first signal passage as the control pressure.

TECHNICAL FIELD

The present invention relates to a valve apparatus for use in ahydraulic drive system for civil engineering and construction machinessuch as hydraulic excavators and cranes, as well as a hydraulic drivesystem equipped with the valve apparatus, and more particularly to avalve apparatus for use in a hydraulic drive system including ahydraulic fluid supply source which has a supply pressure controlfunction such as a load sensing system, and also a hydraulic drivesystem for the valve apparatus.

BACKGROUND ART

In a hydraulic drive system for civil engineering and constructionmachines such as hydraulic excavators and cranes, a flow of a hydraulicfluid supplied from a hydraulic fluid supply source to an actuator iscontrolled by a valve apparatus including a flow control valve.

This type hydraulic drive system uses, as a hydraulic fluid supplysource, means for controlling the supply pressure to be held higher afixed value than the load pressure of the actuator. As disclosed in GB2195745A, one example of such means is a pump regulator which implementsa load sensing system for controlling the pump delivery rate such thatthe delivery pressure of a hydraulic pump is higher a fixed value thanthe load pressure. Because the hydraulic fluid is supplied with the loadsensing system just at a flow rate required by the actuator, undesiredsupply of the hydraulic fluid is reduced, which is advantageous ineconomy. On the other hand, the load sensing system also has theshortcoming that the pump delivery pressure cannot be controlled afterthe intention of an operator because of its dependency on the loadpressure. Therefore, when an inertial load such as a swing of hydraulicexcavators is turned, the pump delivery pressure increases up to thesetting pressure of a main relief valve irrespective of the amount of aflow control valve operated. This raises the problem that anacceleration of the inertial load is maximized and the operator suffersfrom a large shock.

A known one of valve apparatus for use in the hydraulic drive systemimplementing the above load sensing system is disclosed in JP, A,61-88002. This disclosed valve apparatus comprises a flow control valvehaving a supply passage communicating with a hydraulic fluid supplysource, a load passage communicating with an actuator, and a firstmeter-in variable restrictor disposed between the supply passage and theload passage and opened dependent on an operation amount thereof; afirst signal passage branched from the load passage downstream of thefirst variable restrictor and including a restrictor and a check valveallowing a hydraulic fluid to flow toward the load passage; a tankpassage communicating with a reservoir tank; a discharge passage forcommunicating the first signal passage with the tank passage; a secondvariable restrictor provided in the discharge passage and having itsopening variable dependent on the operation amount of the flow controlvalve to produce in the first signal passage a control pressuredifferent from load pressure; and a second signal passage for leadingthe control pressure in the first signal passage to the hydraulic fluidsupply source, the valve apparatus being featured in further comprisinga third signal passage for connecting the first signal passage to theupstream side of the first variable restrictor at a point between thecheck valve and the second variable restrictor, and a restrictordisposed in the third signal passage.

With that valve apparatus, the pressure upstream of the first variablerestrictor is reduced by the restrictor in the third signal passage andthen led to the first signal passage. Thus, the reduced pressure is ledas the control pressure to the hydraulic fluid supply source to performthe load sensing control, so that the pump delivery pressure may becontrolled not depending on the load pressure. Also, by adjustingrespective openings of the restrictor in the first signal passage, therestrictor in the second signal passage, and the restrictor in the thirdsignal passage into the appropriate relationship, the dependency on theload pressure can be assured to some extent in a range above thepredetermined operation amount, so that the flow rate dependent on theoperation amount of the flow control valve is obtained.

In the above valve apparatus, however, since the first signal passage isbranched from the load passage downstream of the first variablerestrictor and includes the restrictor, there occurs a flow of thehydraulic fluid passing from the first signal passage through therestrictor therein to the load passage under a normal condition that theoperation amount of the flow control valve is so increased as to securea predetermined differential pressure across the first variablerestrictor. Accordingly, the control pressure which is produced in thefirst signal passage by reducing the pressure upstream of the firstvariable restrictor is lower than the pressure upstream of the firstvariable restrictor, e.g., the pump pressure, but higher than thepressure downstream of the first variable restrictor, i.e., the loadpressure. Consequently, the differential pressure between the pressureupstream of the first variable restrictor and the control pressure inthe first signal passage becomes smaller than the differential pressureacross the first variable restrictor. Thus, if the differential pressureacross the first variable restrictor is set to a desired value, thedifferential pressure between the pressure upstream of the firstvariable restrictor and the control pressure in the first signal passagewould be smaller than the desired value.

The hydraulic fluid supply source for the load sensing system receives,as an input signal, the differential pressure between the deliverypressure of the hydraulic pump and the aforesaid control pressure tothereby control the delivery rate of the hydraulic pump such that theabove differential pressure becomes equal to a preset target value.Accordingly, the smaller differential pressure between the pressureupstream of the first variable restrictor and the control pressure inthe first signal passage implies that the target value must be set to asmaller one. The reduced target value leads to the problem that thecontrol gain is also reduced and hunting is more likely to occur.

If the differential pressure across the first variable restrictor is setto a larger value, the aforesaid differential pressure as the inputsignal to the hydraulic fluid supply source for the load sensing systemcould be increased. But, the larger differential pressure across thefirst variable restrictor would increase the pressure loss in the firstvariable restrictor and would be undesirable from the standpoint ofeconomy.

An object of the present invention is to provide a valve apparatus and ahydraulic drive system which can control the pump delivery pressure andthe drive pressure of an actuator dependent on the operation amount of aflow control valve, and can increase the differential pressure as aninput signal to a load sensing system, when the actuator is driven.

DISCLOSURE OF THE INVENTION

To achieve the above object, the present invention provides a valveapparatus for controlling a flow of a hydraulic fluid supplied from ahydraulic fluid supply source to an actuator, comprising a flow controlvalve having a supply passage communicating with said hydraulic fluidsupply source, a load passage communicating with said actuator, and afirst meter-in variable restrictor disposed between said supply passageand said load passage and opened dependent on an operation amountthereof; a first signal passage located downstream of said firstvariable restrictor and having a passage section for detecting loadpressure of said actuator; a tank passage communicating with a reservoirtank; a discharge passage for communicating said first signal passagewith said tank passage; and a second variable restrictor provided insaid discharge passage and having its opening variable dependent on theoperation amount of said flow control valve to produce in said firstsignal passage a control pressure different from said load pressure, thecontrol pressure in said first signal passage being led to saidhydraulic fluid supply source through a second signal passage, whereinsaid valve apparatus further comprises auxiliary restrictor meansdisposed in said first signal passage for reducing the load pressuredetected in said passage section of said first signal passage so that apressure lower than the detected load pressure is produced in said firstsignal passage as said control pressure.

The present invention also provides a hydraulic drive systemincorporating the above valve apparatus.

With the present invention thus arranged, since the second variablerestrictor having an opening variable dependent on the operation amountof the flow control valve is disposed in the discharge passage, and theauxiliary restrictor means is disposed in the first signal passage, sothat the load pressure is adjusted by two restrictors; i.e., the secondvariable restrictor and the auxiliary restrictor means, to therebycreate the control pressure, in the sole operation of the abovehydraulic actuator, assuming that the target pressure to be held by theload sensing system implemented with the hydraulic fluid supply sourceis ΔP, the opening area of the first variable restrictor is A, theopening area of the auxiliary restrictor means is a1, and the openingarea of the second variable restrictor is a2, the port pressure of theload passage, i.e., the drive pressure of the hydraulic actuator, is afunction of A, al, a2 and ΔP. Because A and a2 are determined dependenton the operation amount of the flow control valve, the drive pressurecan be obtained dependent on the operation amount of the flow controlvalve. Further, because the hydraulic fluid supply source implements theload sensing system, the pump delivery pressure can also be produceddependent on the operation amount of the flow control valve.

In the combined operation of the above hydraulic actuator and other oneor more actuators, because a pressure compensating valve for controllingthe differential pressure across the first variable restrictor isdisposed, the port pressure of the load passage, i.e., the drivepressure of the hydraulic actuator, is a function of A, al, a2 and ΔP*,assuming that the target pressure to be held by the pressurecompensating valve is ΔP*. As with the above case, the drive pressureand the pump delivery pressure can be both obtained dependent on theoperation amount of the flow control valve.

Accordingly, it is possible to carry out the operation as intended by anoperator with higher accuracy for providing superior operability, and tocontrol an acceleration of an inertial load driven by the hydraulicactuator for alleviating the shock perceived by the operator.

In addition, with the present invention, since the load pressure isintroduced to the first signal passage through the auxiliary restrictormeans to create the control pressure, the control pressure is lower thanthe load pressure, and the differential pressure between the pumpdelivery pressure and the control pressure is larger than thedifferential pressure across the first variable restrictor. Therefore,the differential pressure across the first variable restrictor can beset to a normal small value which results in small pressure loss, sothat the differential pressure between the pump delivery pressure andthe control pressure may be a satisfactorily large value. Consequently,it is possible to increase the control gain of the load sensing systemand achieve stable control of the hydraulic pump free from hunting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a hydraulic drive system incorporating avalve apparatus according to a first embodiment of the presentinvention.

FIG. 2 is a detailed view of a pump regulator used in the hydraulicdrive system of FIG. 1.

FIG. 3 is a characteristic view showing the relationships between thespool stroke of a flow control valve and the opening areas of a firstvariable restrictor, a second variable restrictor and a fixed restrictoras developed in the first embodiment.

FIG. 4 is a diagram schematically showing a hydraulic system including asignal passage and a discharge passage established in the firstembodiment.

FIG. 5 is a vertical sectional view of a valve apparatus according to asecond embodiment of the present invention.

FIG. 6 is a circuit diagram showing the valve apparatus shown in FIG. 5in terms of function.

FIGS. 7(a) and 7(b) are detailed views of a second variable restrictorand a fixed restrictor provided in the valve apparatus shown in FIG. 5.

FIG. 8 is a characteristic view showing the relationships between thespool stroke of a flow control valve and the opening areas of a firstvariable restrictor, the second variable restrictor and the fixedrestrictor as developed in the second embodiment shown in FIG. 5.

FIG. 9 is a vertical sectional view of a valve apparatus according to athird embodiment of the present invention.

FIG. 10 is a vertical sectional view of a valve apparatus according to afourth embodiment of the present invention.

FIG. 11 is a circuit diagram showing the valve apparatus shown in FIG.10 in terms of function.

FIG. 12 is a vertical sectional view of a valve apparatus according to afifth embodiment of the present invention.

FIG. 13 is a schematic view of a hydraulic drive system incorporating avalve apparatus according to a sixth embodiment of the presentinvention.

FIG. 14 is a vertical sectional view of a valve apparatus according to aseventh embodiment of the present invention.

FIG. 15 is a vertical sectional view of a valve apparatus according toan eighth embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

To begin with, a first embodiment of the present invention will bedescribed below with reference to FIGS. 1 through 4. This embodimentpertains to a hydraulic drive system for driving a single-actingactuator.

In FIG. 1, the hydraulic drive system of this embodiment comprises ahydraulic fluid supply source made up by a hydraulic pump 1 of variabledisplacement type and a pump regulator 2 for controlling thedisplacement volume of the hydraulic pump 1 and constituting a loadsensing system, a main relief valve 3 for setting maximum pressure of ahydraulic fluid delivered from the hydraulic pump 1, a single-actingactuator, e.g., a hydraulic motor 4, driven by the hydraulic fluiddelivered from the hydraulic pump 1, and a valve apparatus 5 forcontrolling a flow of the hydraulic fluid supplied from the hydraulicpump 1 to the hydraulic motor 4.

The pump regulator 2 controls the displacement volume of the hydraulicpump 1 such that a differential pressure Pd-PLXmax between a deliverypressure Pd of the hydraulic pump 1 and a later-described maximumcontrol pressure PLXmax, or a differential pressure Pd-PLX between thepump (delivery) pressure Pd and a later-described control pressure PLXassociated with the hydraulic motor 4 in the case of sole operation ofthe hydraulic motor 4, is balanced with preset pressure ΔP. In otherwords, the delivery rate of the hydraulic pump 1 is controlled so as tokeep the relationship of Pd=PLXmax+ΔP.

The pump regulator 2 is detailed in FIG. 2. The pump regulator 2comprises an actuator 50 operatively coupled to a swash plate 1a of thehydraulic pump 1 for controlling the displacement volume of thehydraulic pump 1, and a regulating valve 51 operated in response to thedifferential pressure Pd-PLXmax between the pump pressure Pd and themaximum control pressure PLXmax for controlling operation of theactuator 50. The actuator 50 comprises a double-acting cylinder having apiston 50a having opposite end faces of different pressure receivingareas from each other, and a small-diameter cylinder chamber 50b and alarge-diameter cylinder chamber 50c positioned to receive the oppositeend faces of the piston 50a, respectively. The small-diameter cylinderchamber 50b is communicated with a delivery line 1b of the hydraulicpump 1 through a line 52, whereas the large-diameter cylinder chamber50c is selectively communicated with the delivery line 1b through a line53, the regulating valve 51 and a line 54, or with a reservoir tank 56through the line 53, the regulating valve 51 and a line 55. Theregulating valve 51 has two drive parts 51a, 51b in opposite relation.The pump pressure Pd is loaded to one drive part 51a through a line 57and the line 54, whereas the maximum control pressure PLXmax is loadedto the other drive part 51b through a signal line 19 as a second signalpassage described later. A spring 51c is also disposed in the regulatingvalve 51 on the same side as the driver part 51b.

As the maximum control pressure PLXmax detected by the signal line 19rises, the regulating valve 51 is shifted leftwardly on the drawing totake an illustrated position. In this state, the large-diameter cylinderchamber 50c of the actuator 50 is communicated with the delivery line1b, whereupon the piston 50a is moved leftwardly on the drawing becauseof the difference in pressure receiving area between the opposite endfaces of the piston 50a to increase the tilting amount of the swashplate 1a, i.e., the displacement volume of the hydraulic pump 1. As aresult, the pump delivery rate is increased to raise the pump pressurePd. With the pump pressure Pd raised, the regulating valve 51 isreturned back rightwardly on the drawing. When the differential pressurePd-PLXmax reaches a target value determined by the spring 51c, theregulating valve 51 is stopped and the pump delivery rate is keptconstant. On the contrary, as the maximum control pressure PLXmaxlowers, the regulating valve 51 is shifted rightwardly on the drawing.At this shift position, the large-diameter cylinder chamber 50c of theactuator 50 is communicated with the reservoir tank 56, whereupon thepiston 50a is moved rightwardly on the drawing to decrease the tiltingamount of the swash plate 1a. As a result, the pump delivery rate isdecreased to lower the pump pressure Pd. With the pump pressure Pdlowered, the regulating valve 51 is returned back leftwardly on thedrawing. When the differential pressure Pd-PLXmax reaches the targetvalue determined by the spring 51c, the regulating valve 51 is stoppedand the pump delivery rate is kept constant. In this manner, the pumpdelivery rate is controlled such that the differential pressurePd-PLXmax is held at the target differential pressure determined by thespring 51c.

Returning to FIG. 1, the valve apparatus 5 comprises a flow controlvalve 8 for controlling a flow rate of the hydraulic fluid supplied tothe hydraulic motor 4, a pressure compensating valve 9 disposed upstreamof the flow control valve 8 for controlling the differential pressureacross the flow control valve 8 to supply the hydraulic fluid at asubstantially constant flow rate irrespective of fluctuations in theload pressure PL of the hydraulic motor 4 and the pump pressure Pdduring the combined operation, a supply passage 11 communicating withthe pump 1 through the pressure compensating valve 9, and a load passage12 capable of communicating with the supply passage 11 and connected tothe hydraulic motor 4. The flow control valve 8 comprises a spool madeup of a spool section 7a, a spool section 7b and a rod 7c integrallyformed together. The spool section 7a has formed therein a firstmeter-in variable restrictor 14 having an opening variable dependent onthe operation amount of the flow control valve 8, i.e., the spoolstroke, to disconnect or connect between the supply passage 11 and theload passage 12, and a detection port 15 opened downstream of the firstvariable restrictor 14 for fluid communication with the load passage 12to detect the load pressure of the hydraulic motor 4.

The valve apparatus 5 also comprises a first signal passage (hereinaftersimply referred to as a signal passage) 18 communicating with thedetection port 15, a shuttle valve 10 disposed downstream of the signalpassage 18, a discharge passage 30 branched from the signal passage 18,and a tank passage 13 communicating with the reservoir tank 56. Thespool section 7b of the flow control valve 8 has formed therein a secondvariable restrictor 21 having an opening variable dependent on the spoolstroke to connect or disconnect between the discharge passage 11 and thetank passage 13. The second variable restrictor 21 is configured suchthat it is opened with a predetermined opening when the flow controlvalve 8 is in a neutral position, and is closed after opening of thefirst variable restrictor 14 when the operation amount of the flowcontrol valve 8, i.e., the spool stroke, increases. Further, the signalpassage 18 has a fixed restrictor 22 as auxiliary restrictor meansdisposed between the detection port 15 and the point where the dischargepassage 30 is branched from the signal passage 18.

The second variable restrictor 21 and the fixed restrictor 22 jointlyserve to adjust the load pressure detected by the detection port 15 forcreating the control pressure PLK in the signal passage 18. When thesecond variable restrictor 21 is open, a small amount of the hydraulicfluid flows from the detection port 15 to the tank passage 13 throughthe signal passage 18 and the discharge passage 30. The load pressuredetected by the detection port 15 is reduced by the second variablerestrictor 21 and the fixed restrictor 22 so that the control pressurePLK lower than the load pressure PL is produced downstream of the fixedrestrictor 22 in the signal passage 18. When the second variablerestrictor 21 is closed, there occurs no such a flow of the hydraulicfluid thereby to create the control pressure PLK equal to the loadpressure.

The shuttle valve 10 serves as higher-pressure selector means forselecting maximum one of control pressures including the controlpressure PLK. The selected maximum control pressure PLXmax is passed toa signal line 19 as a second signal passage so that the pump regulator 2is controlled to regulate the displacement volume of the hydraulic pump1 for implementation of the load sensing load sensing system, asmentioned above.

The valve apparatus 5 further comprises passages 31, 32 for leadinginlet pressure Pz of the first variable restrictor 14 and the controlpressure PLK to the pressure compensating valve 9, respectively. Thepressure compensating valve 9 operates so as to hold differentialpressure Pz-PLK between the inlet pressure Pz of the first variablerestrictor 14 and the control pressure PLX at substantially constantdifferential pressure ΔP*. As a result, the differential pressure acrossthe flow control valve 8 is controlled to an almost fixed value.

Shift timing of the first and second variable restrictors 14, 21 of theflow control valve 8 and the detection port 15 with respect to the spoolstroke, as taken place when the spool of the flow control valve 8 ismoved from a neutral position leftwardly in FIG. 1 in theabove-described valve apparatus 5, will now be explained with referenceto a characteristic graph of FIG. 3 showing the relationship between thespool stroke and the respective opening areas. In FIG. 3, acharacteristic line 20a represents the opening area of the secondvariable restrictor 21, a characteristic line 20b represents the openingarea between the detection port 15 and the load passage 12, and acharacteristic line 20c represents the opening area of the firstmeter-in variable restrictor 14. In addition, a characteristic line 20drepresents characteristics of the fixed restrictor 22.

First, as seen from the characteristic line 20a in FIG. 3, when thespool of the flow control valve 8 is in a neutral position, the secondvariable restrictor 21 is open with a predetermined opening, and thecontrol pressure in the signal passage 18 is equal to the tank pressure.When the spool of the flow control valve 8 is moved rightwardly on thedrawing from the above condition, the detection port 15 opens tocommunicate with the load passage 12 so that the load pressure PL of thehydraulic motor 4 shown in FIG. 1 is led to the detection port 15, asseen from the characteristic line 20b in FIG. 3. In this condition, thesecond variable restrictor 21 is still open.

When the spool of the flow control valve 8 is further moved rightwardly,the first meter-in variable restrictor 14 now opens, whereupon thehydraulic fluid supplied through the pressure compensating valve 9 fromthe hydraulic pump 1 shown in FIG. 1 is introduced to the hydraulicmotor 4 through the supply passage 11, the first variable restrictor 14and the load passage 12 shown in FIG. 1. As seen from the characteristicline 20a, at the time when the first variable restrictor 14 opens, thesecond variable restrictor 21 still remains opened, but its opening areahas started decreasing. Afterward, the opening area of the firstvariable restrictor 14 is gradually increased with an increase in thespool stroke, whereas the opening area of the second variable restrictor21 is gradually decreased. Consequently, downstream of the fixedrestrictor 22 in the signal passage 18 shown of FIG. 1, the detectedpressure is adjusted by the fixed restrictor 22 and the second variablerestrictor 21 to create the control pressure PLX lower than the loadpressure PL. The control pressure PLX is passed to the regulating valve51 (see FIG. 2) of the pump regulator 2 through the shuttle valve 10 andthe signal line 19 shown in FIG. 3, as mentioned above, whereby the pump1 is controlled such that the delivery pressure Pd is raised up to avalue given by Pd=PLK+ΔP. As a result, the delivery pressure Pd of thehydraulic pump 1 and the port pressure of the load passage 12, i.e., thedrive pressure (=load pressure) PL of the hydraulic motor 4 can becontrolled as described later.

When the spool is further moved from the above condition, the secondvariable restrictor 21 is closed as seen from the characteristic line20a in FIG. 3, and the control pressure PLX equal to the load pressurePL is created in the signal passage 18. This control pressure is passedto the pump regulator 2, whereby the pump 1 is controlled such that thedelivery pressure Pd is raised up to a value given by Pd=PLX+ΔP. Thehydraulic fluid from the hydraulic pump 1 is supplied to the hydraulicmotor 4 through the pressure compensating valve 9, the supply passage11, the first variable restrictor 14 and the load passage 12 foroperating the hydraulic motor 4 to drive a working member (not shown).

Operation in a range of the spool stroke from opening of the firstvariable restrictor 14 to closing of the second variable restrictor 21,i.e., in a region S1 in FIG. 3, will be explained below. A hydraulicsystem including the first variable restrictor 14, the detection port15, the fixed restrictor 22, the signal passage 18, the dischargepassage 30, the second variable restrictor 21 and the tank passage 13can be schematically depicted as shown in FIG. 4.

Supposing now that only the hydraulic motor 4 is driven solely and thepressure compensating valve 9 serving to compensate for the differentialpressure ΔP* is not operated and is in a full-open state, the supplypressure, i.e., the pump delivery pressure Pd, is equal to the pressureupstream of the first meter-in variable restrictor 14, i.e., the inletpressure Pz. Also, owing to the presence of the first variablerestrictor 14, the fixed restrictor 22 and the second variablerestrictor 21 connected in series to the hydraulic fluid flowing outfrom the tank passage 13 at a flow rate QT, the relationship among theinlet pressure Pz, the port pressure or the load pressure PL, thecontrol pressure LX and the tank pressure PT is expressed by:

    Pz>PL>PLX>PT=0

Let it now be assumed that the opening area of the first variablerestrictor 14 is A, the opening area of the fixed restrictor 22 is al,the opening area of the second variable restrictor 21 is a2, and thehydraulic motor 4 is in a port-blocked state due to the inertial load ofa driven member, the flow rate of the hydraulic fluid passing throughthe first variable restrictor 14 is also QT and, therefore, thefollowing equations hold: ##EQU1## Elimination of QT, etc. from theabove equations (1) through (4) leads to:

    PL={A.sup.2 (a1.sup.2 +a2.sup.2)/a2.sup.2 (a1.sup.2 +A.sup.2)}ΔP(5)(5)

This can be rewritten to:

    PL=[{1+(a2/a1).sup.2 }/(a2/a1).sup.2 (a1.sup.2 /A.sup.2 +1)]ΔP(6)

It will be found from the above equation that the value of the portpressure PL is determined from A, ΔP, a1 and a2. It will be also foundfrom the equation (4) that the value of the pump delivery pressure Pd islikewise determined from A, ΔP, a1 and a2.

When the hydraulic motor 4 and other one or more actuators (not shown)are driven simultaneously, the pressure compensating valve 9 is operatedto hold the differential pressure between the pressure Pz upstream ofthe first variable restrictor 14 and the control pressure PLX at thesetting value ΔP*. By replacing Pz-PL in the above equation (1) withPz-PLX and ΔP in the above equation (4) with ΔP*, therefore, thefollowing equation is obtained:

    PL={A.sup.2 (a1.sup.2 +a2.sup.2)/a2.sup.2 (a1.sup.2 +A.sup.2)}ΔP*(7)

Accordingly, it will be found that during the combined operation, thevalues of the pump delivery pressure Pd and the port pressure PL arealso determined from A, ΔP*, a1 and a2.

As will be apparent from the forgoing equations (5) through (7), thedrive pressure PL of the hydraulic motor 4, i.e., the port pressure, isa function of the opening areas A and a2 which are determined dependenton the spool stroke of the flow control valve 8. Consequently, in eithercase of the sole operation of the hydraulic motor 4 or the combinedoperation of the hydraulic motor 4 and other one or more actuators,there can be obtained the port pressure PL dependent on the operationamount of the flow control valve 8, i.e., the spool stroke.

With the first embodiment thus arranged, the flow rate of the hydraulicfluid can be controlled primarily by the opening area A of the firstmeter-in variable restrictor 14 and, as seen from the equation (6), themaximum value of the port pressure PL can be controlled by the ratio ofthe opening area a2 of the second variable restrictor 21 to the openingarea a1 of the fixed restrictor 22. Therefore, the pressure control andthe flow control both necessary for operation of hydraulic machines canbe optimally set by appropriate selection of the opening areas A, a1 anda2.

Accordingly, it is possible to carry out the operation as intended bythe operator with higher accuracy for providing superior operability,and to control an acceleration of the inertial load driven by thehydraulic motor 4 for alleviating the shock perceived by the operator.

Further, in this embodiment, since the load pressure PL is introduced tothe signal passage through the fixed restrictor 22 to create the controlpressure PLX, there exists the relationship of PL>PLX. In the soleoperation of the hydraulic motor 4, the pressure compensating valve 9 isfully opened to give Pd=Pz, and the differential pressure ΔP=Pd-PLXbetween the pump delivery pressure Pd and the control pressure PLX islarger than the differential pressure ΔP*=Pz-PL across the firstvariable restrictor 14. It is therefore possible to set the differentialpressure across the first variable restrictor 14 to a normal small valuewhich results in the reduced pressure loss, while reserving thedifferential pressure ΔP at a satisfactorily large value.

The regulating valve 51 of the pump regulator 2 receives thedifferential pressure ΔP between the delivery pressure Pd of thehydraulic pump 1 and the control pressure PLX, as an input signal, tocontrol the delivery rate of the hydraulic pump such that thedifferential pressure ΔP becomes equal to the fixed value determined bythe spring 51c. Accordingly, the smaller differential pressure ΔPimplies that the spring 51c must be set to a small setting value. Withthe setting value reduced, the control gain is so reduced that huntingis more likely to occur. With this embodiment, the differential pressureΔP as the input signal of the pump regulator 2 can be set to a largevalue as mentioned above, it is possible to increase the control gainfor enabling stable control of the hydraulic pump 1 free from hunting.

Moreover, in this embodiment, the control pressure PLX is created fromthe load pressure PL using two restrictors; the fixed restrictor 22 andthe second variable restrictor 21. This results in the advantageouseffect that the flow rate of the hydraulic fluid passing through thesignal passage 18 and the discharge passage 30 to the reservoir tank 56can be reduced, and the pressure control can be achieved with smallerenergy loss.

Although the restrictor 22 is fixed one in the above first embodiment,it may be a variable one whose opening is variable dependent on thespool stroke of the flow control valve 8 as will be understood from theforegoing equations (5) through (7). This modification can furtherimprove control characteristics.

While the spool of the flow control valve 8 comprises the spool sections7a, 7b and the rod 7c integrally formed together, the rod 7c may be madeas a separate member. Alternatively, the spool sections 7a, 7b may bearranged to be movable independently and driven by a common pilotpressure. In addition, either one or both of the first and secondvariable restrictors 14, 21 may be in the form of a poppet valve.

Second Embodiment

A second embodiment of the present invention will be described withreference to FIGS. 5 through 8. This embodiments provides a valveapparatus for driving a double-acting actuator. FIG. 5 is a verticalsectional view of the valve apparatus, and FIG. 6 is a circuit diagramshowing the valve apparatus in terms of function. In these drawings, theidentical components to those shown in FIG. 1 are denoted by the samereference numerals.

In FIGS. 5 and 6, a valve apparatus 5A of this embodiment comprises ablock 6 forming a body, a flow control valve 8A having a spool 7slidable in a spool bore 6a defined in the block 6, a pressurecompensating valve 9 provided upstream of the flow control valve 8A tocontrol differential pressure between inlet pressure Pz and outletpressure PL of the flow control valve 8A, i.e., differential pressurePz-PL across the flow control valve 8A, and a shuttle valve 10 provideddownstream of the flow control valve 8A.

The block 6 has formed therein two supply passages 11a, 11bcommunicating with a hydraulic pump 1, two load passages 12a, 12bcapable of communicating with the supply passages 11a, 11b,respectively, and connected to a hydraulic actuator shown in FIG. 6,e.g., a swing motor 4A for driving a swing of a hydraulic excavator, andtwo tank passages 13a, 13b capable of communicating with the loadpassages 12a, 12b, respectively. The spool 7 has two first meter-invariable restrictors 14a, 14b for communicating the supply passage 11awith the load passage 12a and communicating the supply passage 11b withthe load passage 12b, respectively, and being opened dependent on thestroke of the spool 7, two detection ports 15a, 15b capable of beingopen to the load passages 12a, 12b downstream of the first variablerestrictors 14a, 14b, respectively, to detect the load pressure PL ofthe swing motor 4A, two passages 16a, 16b communicating with thedetection ports 15a, 15b, respectively, and two passages 17a, 17bcommunicating with the passages 16a, 16b, respectively. The block 6further has a passage 18 capable of communicating with the passages 17a,17b.

The spool 7 is also formed with a second variable restrictor 21apositioned between the passage 17b and the passage 18 and having itsopening area variable dependent on the stroke of the spool 7 when thespool 7 is moved rightwardly on the drawing, a second variablerestrictor 21b positioned between the passage 17a and the passage 18 andhaving its opening area variable dependent on the stroke of the spool 7when the spool 7 is moved leftwardly on the drawing, a fixed restrictor22a positioned between the passage 17a and the passage 18 and carryingout its function when the spool 7 is moved rightwardly on the drawing,and a fixed restrictor 22b positioned between the passage 17b and thepassage 18 and carrying out its function when the spool 7 is movedleftwardly on the drawing.

As with the first embodiment, the second variable restrictors 21a, 21bare configured such that they are open at a predetermined opening whenthe spool 7 is in a neutral position, and are closed after opening ofthe first variable restrictors 14a, 14b when the spool stroke isincreased.

The detection port 15a, the passages 16a, 17a and the passage 18 jointlyconstitute a first signal passage for detecting the load pressure of theswing motor 4A downstream of the first variable restrictor 14a, when thespool 7 is moved rightwardly on the drawing. The detection port 15b, thepassages 16b, 17b and the passage 18 jointly constitute a first signalpassage for detecting the load pressure of the swing motor 4A downstreamof the first variable restrictor 14b, when the spool 7 is movedleftwardly on the drawing. Further, the detection port 15b and thepassages 17b, 16a jointly constitute a discharge passage forcommunicating the first signal passage 15a, 16a, 17a, 18 establishedwhen the spool 7 is moved rightwardly on the drawing, with the tankpassage 13b, the second variable restrictor 21abeing disposed in thisdischarge passage. The detection port 15a and the passages 17a, 16ajointly constitute a discharge passage for communicating the firstsignal passage 15b, 16b, 17b, 18 established when the spool 7 is movedleftwardly on the drawing, with the tank passage 13a, the secondvariable restrictor 21b being disposed in this discharge passage.

The fixed restrictor 22a is disposed in the first signal passage 15a,16a, 17a, 18 established when the spool 7 is moved rightwardly on thedrawing, and serves as auxiliary restrictor means for reducing the loadpressure detected by that first signal passage to create the controlpressure PLX lower than the load pressure. The fixed restrictor 22b isdisposed in the first signal passage 15b, 16b, 17b, 18 established whenthe spool 7 is moved leftwardly on the drawing, and serves as auxiliaryrestrictor means for reducing the load pressure detected by that firstsignal passage to create the control pressure PLX lower than the loadpressure.

The control pressure PLX produced in the passage 18 constituting a partof the first signal passage is, similarly to the first embodiment,introduced to a signal line 19 as a second signal passage through theshuttle valve 10 as higher-pressure selector means, and used for theload sensing control by the pump regulator 2.

The second variable restrictors 21a, 21b and the fixed restrictors 22a,22b are detailed in FIGS. 7(a) and 7(b). Of these drawings, FIG. 7(a)shows a neutral state of the spool 7, and FIG. 7(b) shows a state inwhich the spool 7 has been moved leftwardly. Arrows in FIG. 7(b)indicate a flow of the hydraulic fluid in the signal passage and thedischarge passage.

Shift timing of the first and second variable restrictors 14a, 14b and21a, 21b and the detection ports 15a, 15b with respect to the spoolstroke of the flow control valve 8A is shown in FIG. 8. Characteristicsof the first variable restrictors 14a, 14b, i.e., the relations of theiropening areas with respect to the stroke of the spool 7, are setidentical to the characteristic line 20c in FIG. 3. Characteristics ofthe second variable restrictors 21a, 21b are set identical to thecharacteristic line 20a in FIG. 3. Characteristics of the fixedrestrictors 22a, 22b are set identical to the characteristic line 20d inFIG. 3. The opening areas between the detection ports 15a, 15b and theload passages 12a, 12b are set identical to the characteristic line 20bin FIG. 3. In addition, the characteristic line 20e indicates theopening area between the detection ports 15a, 15b and the tank passages13a, 13b.

The swing motor 4A is a double-acting actuator. In a main line connectedto the load passages 12a, 12b of the valve apparatus 5A, there isdisposed a counter balance valve 35 for blocking off the holdingpressure produced when the swing (not shown) is installed on a slope.

With the second embodiment thus arranged, when the spool 7 is moved froma neutral position rightwardly in FIG. 5 with an intention of drivingthe swing motor 4A solely, the communication between the detection port15a and the tank passage 13a is first cut of as seen from thecharacteristic line 20e in FIG. 8. When the spool 7 is further movedfrom the above condition, the first variable restrictors 14a, 14b, thesecond variable restrictors 21a, 21b, the fixed restrictors 22a, 22b,the detection ports 15a, 15b, and the load passages 12a, 12b, thougheach provided in pair, exhibit their characteristics identical to thosein the foregoing first embodiment. Accordingly, the above-describedequations (5) through (7) are also satisfied in the second embodiment.As a result, the port pressure, i.e., the drive pressure PL, and thedelivery pressure Pd of the hydraulic pump 1 can be controlled dependenton the operation amount of the flow control valve 8A, i.e., the spoolstroke, thereby providing the similar advantageous effect to that in thefirst embodiment.

Because the control pressure PLX created in the passage 18 through thefixed restrictors 22a, 22b meets the relationship of PL>PLX, thedifferential pressure ΔP=Pd-PLX between the pump delivery pressure Pdand the control pressure PLX can be a satisfactorily large value.Further, because the control pressure PLX is created using tworestrictors; the fixed restrictor 22a, 22b and the second variablerestrictor 21a, 21b, the flow rate of the hydraulic fluid passing fromthe detection port 15a, 15b as the signal passage to the tank passage13b, 13a through the passage 18 and the detection port 15b, 15a as thedischarge passage can be reduced, and the pressure control can beachieved with smaller energy loss. In this point, the similaradvantageous effect to that in the first embodiment can also beobtained.

It is of course a matter that although the restrictors 22a, 22b arefixed ones in this embodiment, they may be variable ones whose openingsare variable dependent on the stroke of the spool 7, as with theforegoing first embodiment.

Third Embodiment

A third embodiment of the present invention will be described withreference to FIG. 9. This embodiment is to give the valve apparatus witha function of reserving the holding pressure of the actuator.

In FIG. 9, a valve apparatus 5B of this embodiment has second variablerestrictors 21a, 21b and fixed restrictors 22a, 22b identical to thosein the foregoing second embodiment. A check valve 23 with small springpressure is slidably fitted in a spool 7 which constitutes a flowcontrol valve 8B. When the spool 7 is in the vicinity of a neutralposition, the passage 16a is connected to the tank passage 13a throughthe check valve 23, thereby forming the discharge passage. When thespool 7 is moved rightwardly on the drawing, the fixed restrictor 22afunctions between the detection port 15a and the passage 18, and thesupply passage 11a is communicated with the load passage 12a through thecheck valve 23 upon opening of the first meter-in variable restrictor14a. When the spool 7 is moved leftwardly on the drawing, the passage 18is communicated with the tank passage 13a through the second variablerestrictor 21b, the passage 17a, the passage 16a and the check valve 23which jointly define the discharge passage.

Then, as an actuator of which operation is controlled by the valveapparatus 5B, there is provided a hydraulic cylinder, e.g., a boomcylinder 4B for driving a boom of hydraulic excavators. The boomcylinder 4B is communicated at the head side with the load passage 12ain which the check valve 23 is located, and at the rod side with theload passage 12b.

During operation of a boom (not shown) carried out by the boom cylinder4B, for example, when the boom is held at an elevated level in air, thedead load of the boom acts on the boom cylinder 4B and the holdingpressure is produced in the head side line of the boom cylinder 4B,i.e., the load passage 12a.

With the third embodiment thus arranged, when the spool 7 of the flowcontrol valve 8B is moved rightwardly with an intention of driving theboom cylinder 4B solely, the detection port 15a is first disconnectedfrom the tank passage 13a, and the detection port 15a is thencommunicated with the load passage 12a. Afterward, the passage 16a iscommunicated with the supply passage 11a through the first meter-invariable restrictor 14a. Consequently, the first variable restrictor14a, the fixed restrictor 22a and the second variable restrictor 21a nowconstitute the foregoing hydraulic system shown in FIG. 4. As a result,the above-described equations (5) through (7) are also satisfied in thethird embodiment, whereby the port pressure PL and the pump deliverypressure can be controlled dependent on the spool stroke of the flowcontrol valve 8B as with the foregoing second embodiment. At this time,the hydraulic fluid is supplied from the supply passage 11a to the headside of the boom cylinder 4B through the first variable restrictor 14a,the passage 16a, the check valve 23 and the load passage 12a.

In this connection, if the aforesaid holding pressure is produced in thehead side line of the boom cylinder 4B, i.e., the load passage 12a, thepressure in the passage 16a is determined by the stroke of the spoolwithin the stroke range where the hydraulic system shown in FIG. 4 isestablished, and that pressure may be lower than the holding pressureproduced in the load passage 12a. To cope with that, in this embodiment,the check valve 23 acts to prevent the hydraulic fluid from flowing fromthe load passage 12a the passage 16a. Therefore, even if the holdingpressure is produced in the head side line of the boom cylinder 4B,i.e., the load passage 12a, the hydraulic fluid under pressure held inthe load passage 12a will not flow into the passage 16a and then flowout to the reservoir tank through the fixed restrictor 22a, the passage18, the second variable restrictor 21a, and the discharge passage whichis defined by the passages 17b, 16b and the detection port 15b.Consequently, this embodiment can reserve a holding function to preventcontraction of the boom cylinder 4B, i.e., a drop of the boom by thegravity or dead load.

On the contrary, when the spool 7 of the flow control valve 8 is movedleftwardly, the supply passage 11b is communicated with the load passage12b in which no holding pressure occurs, through the first meter-invariable restrictor 14b and the passage 16b. Also, the second variablerestrictor 21a, the passages 17a, 16a, the check valve 23 and thedetection port 15a jointly define the discharge passage led to the tankpassage 13a. In this embodiment, therefore, since the hydraulic systemshown in FIG. 4 is established by the fixed restrictor 22b and thesecond variable restrictor 21b, the foregoing equation (5) through (7)are satisfied and the port pressure PL and the pump delivery pressurecan be controlled desirably. At this time, the returning hydraulic fluidon the head side of the boom cylinder 4B is discharged from the loadpassage 12a to the tank passage 13a through the passages 24, 16a and thecheck valve 23.

Thus, with satisfaction of the foregoing equations (5) through (7), thethird embodiment can control the port pressure (drive pressure) PL andthe pump delivery pressure dependent on the spool stroke of the flowcontrol valve 8B, and can achieve force control for regulating thrust ofthe boom cylinder 4B with the control of the port pressure.

In addition, since the third embodiment includes the check valve 23between the load passage 12a and the first variable restrictor 14a, whenthe spool 7 shown in FIG. 9 is moved rightwardly to extend the boomcylinder 4B, the hydraulic fluid held under pressure on the head side ofthe boom cylinder 4B will not flow into the passage 16a, and the boom(not shown) can be prevented from dropping by the dead load uponcontraction of the boom cylinder 4B.

Fourth Embodiment

A fourth embodiment of the present invention will be described withreference to FIGS. 10 and 11. This embodiment is to provide a valveapparatus for use in a double-acting actuator which has no counterbalance valve.

In FIG. 10, a valve apparatus 5C includes a pair of check valves 25a,25b disposed in a spool 7 of the flow control valve 8C. The check valve25a is disposed between the supply passage 11a and the load passage 12aas well as the tank passage 13a, while the check valve 25b is disposedbetween the supply passage 11b and the load passage 12b as well as thetank passage 13b. A swing motor 4A having no counter balance valve isprovided as an actuator to drive a swing (not shown).

The spool 7 of the flow control valve 8C is depicted as shown in FIG. 11in terms of function. When the spool 7 is moved rightwardly from thecondition shown in FIG. 11, a region S1 of this spool 7 corresponds tothe aforesaid region S1 in FIG. 8, i.e., the stroke region where thefixed restrictor 22a and the second variable restrictor 21a bothfunction as restrictors. Also, a region S2 of the spool 7 shown in FIG.11 corresponds to the aforesaid region S2 in FIG. 8, i.e., the strokeregion where the second variable restrictor 21a is closed. The remainingstructure of the valve apparatus 5C is identical to that shown in FIG.9.

With the fourth embodiment thus arranged, when the spool 7 of the flowcontrol valve 8C is moved rightwardly in FIGS. 10 and 11, for example,the hydraulic system shown in FIG. 4, which includes the first variablerestrictor 14a, the fixed restrictor 22a, and the discharge passagehaving the second variable restrictor 21a and the check valve 25therein, is established in a range of the region S1 shown in FIG. 11.Therefore, the foregoing equations (5) through (7) are satisfied and theport pressure PL can be controlled dependent on the stroke of the spool7, i.e., the lever operation amount of the flow control valve 8C in anyoperation of driving the swing motor solely or in combination with otherone or more actuators. This is equally applied to the case where thespool 7 is moved leftwardly in FIGS. 10 and 11. As a result, the similaradvantageous effect to that in the foregoing second embodiment can beobtained.

Furthermore, if the swing (not shown) is installed on a slope, forexample, the holding pressure is produced in either the load passage 12aor 12b both connected to the swing motor 4A. In such a case, when thespool 7 of the flow control valve 8C is moved, the hydraulic systemshown in FIG. 4 is established in a range of the region S1 shown in FIG.11 as mentioned above, and the pressure in the passage 16a or 16b isdetermined by the stroke of the spool 7, resulting in that the pressurein the passage 16a or 16b may be lower than the holding pressureproduced in the load passage 12a, 12b. With this fourth embodiment,however, no matter which one of the load passages 12a, 12b is subjectedto the holding pressure, the hydraulic fluid held in the load passageunder pressure is prevented from flowing into the supply passage 11a,11b by the corresponding one of the check valves 25a, 25b. This ensuresit to avoid operation of the swing motor 4A not intended by theoperator, i.e., undesired motion of the swing (not shown).

Fifth Embodiment

A fifth embodiment of the present invention will be described withreference to FIG. 12. This embodiment has an operator check, in place ofthe check valve, to block off the holding pressure.

In FIG. 12, a valve apparatus 5D of this embodiment has an operatorcheck 26 in a load passage 12a which is defined in a block 6constituting the valve apparatus body and is subjected to the holdingpressure of a boom cylinder 4B. The remaining structure is identical tothat of the second embodiment shown in FIG. 5.

With this fifth embodiment thus arranged, the foregoing equations (5)through (7) are satisfied on the basis of the hydraulic system includingthe first variable restrictors 14a, 14b, as well as the correspondingfixed restrictors 22a, 22b and the second variable restrictors 21a, 21b.Therefore, the port pressure PL and the pump delivery pressure can becontrolled dependent on the lever operation amount of the flow controlvalve 8B. In addition, when the hydraulic fluid is supplied to the loadpassage 12a to extend the boom cylinder 4B, the operator check 26 isopened only after the pressure in the load passage 12a becomes largerthan the holding pressure acting on the head side of the boom cylinder4B, allowing the hydraulic fluid to be supplied to the head side of theboom cylinder 4B for driving of the boom cylinder 4B. Consequently, thehydraulic fluid boosted in pressure for holding the boom cylinder 4B isprevented from flowing into the supply passage 11a, and the similaradvantageous effect to that in the third embodiment of FIG. 9 can beobtained.

Sixth Embodiment

A sixth embodiment of the present invention will be described withreference to FIG. 13. A valve apparatus 5E according to the sixthembodiment, shown in FIG. 13, has a limited 36 for limiting theoperation amount of a flow control valve 8E to a predetermined amount inshort of the maximum stroke, in addition to the structure of theforegoing first embodiment shown in FIG. 1. The limiter 36 comprises,for example, a projection against which a spool section 7a of the flowcontrol valve 8E strikes for restriction of its movement. A maximumvalue of the stroke restricted by the limiter 36 corresponds to a pointX contained in the region S1 of FIG. 3 by way of example.

The sixth embodiment thus arranged is effective in the case where theinertial load to be driven by the hydraulic motor 4 is relatively smalland, therefore, the load pressure is small. The installed position ofthe limiter 36 is previously set such that when the flow control valve8E is operated until the spool section 7a strikes against the limiter36, the load pressure PL determined by the foregoing equations (5)through (7) has a value substantially in agreement with the drivepressure necessary for the hydraulic motor 4. With such presetting, themaximum port pressure is determined from the above equation (6), and theload pressure applied to the hydraulic motor is limited to therelatively small load pressure PL corresponding to the point X in FIG.3.

Accordingly, with this sixth embodiment, since the basic structure isidentical to that of the foregoing first embodiment, the aforesaidequations (5) through (7) are satisfied, whereby the flow rate and theload pressure PL can be controlled as intended by the operator. Inaddition, without the need of especially installing a relief valveadapted to release the surplus load pressure produced in a circuitcontaining the hydraulic motor 4, it is possible to protect equipment inthat circuit from damage, and to suppress energy loss which wouldotherwise be caused with release of the surplus load pressure, resultingin an advantage of economy.

Seventh Embodiment

A seventh embodiment of the present invention will be described withreference to FIG. 14. A valve apparatus 5F according to the seventhembodiment, shown in FIG. 14, has a limiter 36A in addition to thestructure of the foregoing second embodiment shown in FIG. 5. Thelimiter 36A comprises a screw 37 for limiting the stroke of a spool 7 ofa flow control valve 8F to a predetermined position in short of themaximum stroke, and a lock nut 38 for fastening the screw 37 in place.

As with the foregoing sixth embodiment, this seventh embodiment can alsolimit the drive pressure of the actuator to be controlled by the valveapparatus 5F, and provide the similar advantageous effect to that in thesixth embodiment.

Eighth Embodiment

An eighth embodiment of the present invention will be described withreference to FIG. 15. A valve apparatus 5G according to the eighthembodiment has a pilot valve 39 and a pressure reducing valve 36B forreducing pilot pressure generated by the pilot valve 39. The pressurereducing valve 36B serves as a limiter for limiting the operation amountof a spool 7 of a flow control valve 8G. The remaining structure isidentical to that of the foregoing second embodiment shown in FIG. 5.

Thus, by adjusting the pilot pressure, it is also possible to achievethe similar operation to that in the foregoing seventh embodiment, andto provide the similar advantageous effect to that in the seventhembodiment.

With the pressure reducing valve 36B as a limiter being in the form of asolenoid proportional valve, the maximum pilot pressure, i.e., themaximum stroke, can be adjusted using an electric signal.

Industrial Applicability

According to the present invention, when the flow control valve isoperated from a neutral position in the sole or combined operation ofone or more actuators, the delivery pressure of the hydraulic pump andthe drive pressure of the actuator can be controlled dependent on theoperation amount of the flow control valve. This reliably eliminates theevent that the pump delivery pressure may be increased up to the settingpressure of a main relief valve against the intention of an operator,and ensures excellent operability. Also, the control of the drivepressure permits force control of the actuator so that, when theactuator drives an inertial load, an acceleration of the inertial loadmay be controlled. As a result, the shock perceived by the operator canbe alleviated.

Further, since the load pressure is reduced by a fixed restrictor tocreate the control pressure, the differential pressure between the pumpdelivery pressure and the control pressure can be set to asatisfactorily large value to thereby enable the loading sensing controlof the hydraulic pump free from hunting. In addition, since the controlpressure is created using two restrictors; i.e., the fixed restrictorand the second variable restrictor, the flow rate of the hydraulic fluidflowing from the signal passage to the reservoir tank through thedischarge passage can be reduced so as to achieve the pressure controlwith small energy loss.

What is claimed is:
 1. A valve apparatus for controlling a flow of ahydraulic fluid supplied from a hydraulic fluid supply source to anactuator, comprising a flow control valve having a supply passagecommunicating with said hydraulic fluid supply source, a load passage;communicating with said actuator, and a first meter-in variablerestrictor disposed between said supply passage and said load passageand opened dependent on an operation amount thereof; a first signalpassage located downstream of said first variable restrictor and havinga passage section for detecting load pressure of said actuator; a tankpassage communicating with a reservoir tank; a discharge passage forcommunicating said first signal passage with said tank passage; and asecond variable restrictor provided in said discharge passage and havingits opening variable dependent on the operation amount of said flowcontrol valve to produce in said first signal passage a control pressuredifferent from said load pressure, the control pressure in said firstsignal passage being led to said hydraulic fluid supply source through asecond signal passage, wherein:said valve apparatus further comprisesauxiliary restrictor means disposed in said first signal passage forreducing the load pressure detected in said passage section of saidfirst signal passage so that a pressure lower than the detected loadpressure is produced in said first signal passage as said controlpressure.
 2. A valve apparatus according to claim 1, wherein said secondvariable restrictor is configured to be open to a predetermined openingwhen said flow control valve is in a neutral position, and closed afteropening of said first variable restrictor when said flow control valveis operated.
 3. A valve apparatus according to claim 1, furthercomprising higher-pressure selector means for selecting a maximum one ofcontrol pressures including the control pressure produced in said firstsignal passage, and leading the selected maximum pressure as the controlpressure to said second signal passage.
 4. A valve apparatus accordingto claim 1, further comprising a pressure compensating valve forcontrolling a differential pressure across said first variablerestrictor, and a third signal passage for leading the control pressureproduced in said first signal passage to said pressure compensatingvalve, wherein said pressure compensating valve holds a differentialpressure between an inlet pressure of said first variable restrictor andthe control pressure in said first signal passage at a predeterminedvalue to thereby control the differential pressure across said firstvariable restrictor.
 5. A valve apparatus according to claim 1, whereinsaid flow control valve has a spool movable in its axial direction, andsaid first variable restrictor, said second variable restrictor and saidauxiliary restrictor means are formed in said spool.
 6. A valveapparatus according to claim 1, wherein a check valve is disposedbetween said first variable restrictor and said load passage forallowing the hydraulic fluid to flow only in a direction toward saidload passage from said first variable restrictor.
 7. A valve apparatusaccording to claim 1, wherein an operator check is disposed in said loadpassage.
 8. A valve apparatus according to claim 1, further comprisinglimiter means for limiting the operation amount of said flow controlvalve to a predetermined value.
 9. A valve apparatus for controlling aflow of hydraulic fluid supplied from a hydraulic fluid supply source toa double-acting actuator, comprising a flow control valve having supplypassages communicating with said hydraulic fluid supply source, a pairof load passages communicating with said actuator, and a pair of firstmeter-in variable restrictors disposed between said supply passages andsaid pair of load passages, respectively, and opened alternativelydependent on the operating direction to an opening dependent on anoperation amount thereof; a pair of first signal passages locateddownstream of said pair of first variable restrictors, respectively, andhaving passage sections for detecting load pressure of said actuatoralternatively dependent on the operating direction; a pair of tankpassages each communicating with a reservoir tank; a pair of dischargepassages for communicating said pair of first signal passages with saidpair of tank passages, respectively; and a pair of second variablerestrictors provided in said pair of discharge passages, respectively,and having their openings variable dependent on the operation amount ofsaid flow control valve to produce in said pair of first signal passagesa control pressure different from the load pressure detected in thecorresponding first signal passage, alternatively dependent on theoperating direction, the control pressure produced alternatively in saidpair of first signal passages being led to said hydraulic fluid supplysource through a second signal passage, wherein:said valve apparatusfurther comprises a pair of auxiliary restrictor means disposed in saidpair of first signal passages, respectively, for reducing the loadpressure detected alternatively in said passage sections of said pair offirst signal passages so that a pressure lower than the detected loadpressure is produced in the corresponding first signal passage as saidcontrol pressure.
 10. A valve apparatus according to claim 9, whereinsaid flow control valve has a spool movable in its axial direction, andsaid pair of first variable restrictors, said pair of second variablerestrictors and said pair of auxiliary restrictor means are formed insaid spool.
 11. A valve apparatus according to claim 10, wherein saidspool has a pair of inner passages, one of said pair of inner passagesfunctioning as one of said pair of first signal passages and the otherof said pair of inner passages functioning as one of said pair ofdischarge passages when one of said pair of first variable restrictorsis opened upon said spool axially moving in one direction, one of saidpair of inner passages functioning as the other of said pair ofdischarge passages and the other of said pair of inner passagesfunctioning as the other of said pair of first signal passages when theother of said pair of first variable restrictors is opened upon saidspool axially moving in the other direction.
 12. A valve apparatusaccording to claim 11, wherein said pair of inner passages have firstpassage sections positioned downstream of said pair of first variablerestrictors and second passage sections capable of communicating saidpair of load passages with said pair of tank passages, respectively, andwherein check valves are disposed between said first passage sectionsand said second passage sections, respectively, for allowing thehydraulic fluid to flow only in a direction toward said second passagesections from said first passage sections.
 13. A hydraulic drive systemcomprising a hydraulic fluid supply source, at least one actuator drivenby a hydraulic fluid from said hydraulic fluid supply source, and avalve apparatus for controlling a flow of the hydraulic fluid suppliedfrom said hydraulic fluid supply source to said actuator, said valveapparatus comprising a flow control valve having a supply passagecommunicating with said hydraulic fluid supply source, a load passagecommunicating with said actuator, and a first meter-in variablerestrictor disposed between said supply passage and said load passageand opened dependent on an operation amount thereof; a first signalpassage located downstream of said first variable restrictor and havinga passage section for detecting a load pressure of said actuator; a tankpassage communicating with a reservoir tank; a discharge passage forcommunicating said first signal passage with said tank passage; a secondvariable restrictor provided in said discharge passage and having itsopening variable dependent on the operation amount of said flow controlvalve to produce in said first signal passage a control pressuredifferent from said load pressure; and a second signal passage forleading the control pressure in said first signal passage to saidhydraulic fluid supply source, wherein:said valve apparatus furthercomprises auxiliary restrictor means disposed in said first signalpassage for reducing the load pressure detected in said passage sectionof said first signal passage so that a pressure lower than the detectedload pressure is produced in said first signal passage as said controlpressure.
 14. A hydraulic drive system according to claim 13, whereinsaid hydraulic fluid supply source has a hydraulic pump and pump controlmeans for controlling a delivery rate of said hydraulic pump such that adifferential pressure between a delivery pressure of said hydraulic pumpand the control pressure led through said second signal passage is heldsubstantially constant.
 15. A hydraulic drive system according to claim13, further comprising a pressure compensating valve for controlling adifferential pressure across said first variable restrictor, and a thirdsignal passage for leading the control pressure produced in said firstsignal passage to said pressure compensating valve, wherein saidpressure compensating valve holds a differential pressure between aninlet pressure of said first variable restrictor and the controlpressure in said first signal passage at a predetermined value tothereby control the differential pressure across said first variablerestrictor.